The present invention relates to lightweight core composite articles, and more particularly to the manufacture of such articles wherein the core thereof includes a plurality of protruding pins which pierce both upper and lower core composite face sheets.
Composite articles incorporating honeycomb (HC) cores are commonly utilized for fabricating aerospace structures due to their advantageous strength to weight ratio. Honeycomb core composite articles include upper and lower composite skins, i.e., fiber reinforced resin matrix laminates that are separated and stabilized by the HC core. The high strength and low weight of such sandwich construction results in lower overall aircraft system weight.
HC composite articles may be fabricated utilizing various conventional molding techniques. One commonly employed technique involves the use of a vacuum bag assembly wherein an impervious membrane or xe2x80x9cvacuum bagxe2x80x9d is employed for consolidating the composite skins and ensuring proper adhesion thereof to the centrally disposed honeycomb core. Film adhesive, which is applied to the honeycomb core prior to lay-up, forms the bonds between the upper and lower composite laminates and the honeycomb core. The vacuum bag is disposed over a rigid mold member and sealed thereto so as to form a mold cavity which is occupied by the uncured composite lay-up. The mold cavity is then evacuated and additional pressure and temperature are applied via an autoclave to cure the lay-up. The combination of vacuum and external pressure functions to consolidate the composite skins, remove air and volatiles from the resin binder, and apply the necessary compaction pressure to ensure full and uniform adhesion of the lay-up.
HC composite articles may also be fabricated utilizing fluorelastomeric tooling or xe2x80x9cblack bagxe2x80x9d caul plate fabrication techniques. Black bag fabrication typically includes laying up the composite laminates and the HC core between a rigid mold member and a semi-rigid mold member. The semi-rigid mold member provides uniform pressure transfer to the HC core composite article during co-cure. The entire mold assembly is then vacuum-bagged and co-cured under pressure and temperature in an autoclave as described in the vacuum bag method.
Difficulties commonly encountered during the fabrication of HC composite articles relate to distortion of the HC core under compaction pressure. Such distortion is of particular difficulty for ramped HC cores. An option to minimize distortion includes applying expanding adhesive foam into the HC core to improve stabilization in the ramped area. Stabilization through filler, however, may not be practical for applications wherein the minimization of overall aircraft system weight is critical.
Another option is to limit the co-cure pressure. Generally, limiting the co-cure pressure to a maximum value of about 45 psi significantly reduces the number of incidents of core crushing in fabricating ramped HC core composite articles. Limiting the maximum co-cure pressure, however, is generally unacceptable as the HC core composite articles fabricated utilizing low co-cure pressures may embody an unacceptable level of voids which may reduce the strength of the composite article.
Recently, advanced sandwich core materials are replacing conventional HC cores to separate and stabilize composite skins. One such advanced sandwich core material is X-COR(trademark) manufactured by Aztex, Inc. of Waltham, Mass. Although providing lighter weight, greater damage resistance, and other advantages, such core materials pose particular challenges for conventional composite articles fabrication methods.
Accordingly, it is desirable to provide a fabrication method for lightweight composite articles utilizing advanced core materials. It is further desirable to provide fabrication methods utilizing advanced core materials which are compatible with high co-cure pressures to obtain optimum fiber volume and, consequently, the strength of the composite laminate without crushing the underlying core material.
The composite molding apparatus according to the present invention provides a rigid base member and a complementary semi-rigid mold member or xe2x80x9cblack bagxe2x80x9d caul plate having a sieve member formed therein. The semi-rigid mold member is operative to provide uniform pressure transfer during the high pressure co-cure molding process of the present invention.
The composite article manufactured by the present invention includes uncured composite skins (pre pregs) and a sandwich core such as X-COR(trademark) manufactured by Aztex, Inc. of Waltham, Mass. The sandwich core includes pins which are inserted into a lightweight carrier such as a foam material to form a truss-like structure. The pins protrude from the carrier and include sharpened ends which assist in piercing the prepregs such that the pins form a bond/mechanical lock with the skins during the autoclave cure cycle.
The sieve member of the semi-rigid mold member such as a tight weave stainless steel screen engages the sharpened ends of the pins to limit penetration into the semi-rigid mold member to a predetermined depth during co-cure. The flexible nature of the semi-rigid mold member allows the composite skins to be consolidated down on the X-COR(trademark) material and mechanically control the amount of compaction to the length of the pins in the core while accommodating minor variations in the core material dimensional tolerances without crushing the carrier.
The composite articles fabricated according to the present invention yield similar and higher strength results than corresponding honeycomb core panels at a lighter weight.
Composite articles according to the present invention are lighter than the equivalent strength HC core in part to due to the elimination of film adhesive commonly required to bond skins to the HC core material. Further, as the pins are engaged between the sieve member of the semi-rigid mold member and the rigid mold member, pressures greater than 45 psi may be applied to the lay-up without crushing the core. An increased fiber to resin ratio, and, consequently, the strength of the composite laminate is increased.